1. Field of the Invention
The present invention relates to a glare-resistant touch panel, and more particularly relates to a technique for improving the operability of the touch panel under relative high temperature conditions.
2. Description of the Prior Art
A typical touch panel is constructed of an ultrathin glass plate (base sheet member) and a flexible transparent resin film (top sheet member) that are set facing each other and have resistive membranes respectively on their opposed surfaces, with spacers being placed in between the two sheet members in order to provide a certain spacing.
With the increasing range of uses for such touch panels, the demand for products that deliver new additional values is growing in recent years. For instance, for combined use with a liquid crystal display (LCD) panel equipped in an outdoor use device such as a car navigation system, a glare-resistant touch panel which is coated with a film of polarizer on its surface is devised in order to keep the visibility of the display content of the LCD panel from decreasing due to irregular reflections of incident light.
The polarizer used here is generally made of a resin film. Such a polarizing resin film has a thermal expansion coefficient six to eight times that of glass which forms the base sheet member.
This large difference in thermal expansion coefficient between the top and base sheet members causes the following problems. As the temperature rises, the whole top sheet member including the polarizer bulges outward, thereby seriously deforming the touch panel. Besides, the operability of the touch panel is impaired, since the space between the top and base sheet members widens and so the user has to apply a greater pressure to operate the touch panel. Given the recent trend toward larger touch panels with upsizing of LCD panels, the amount of outward bulge of the top sheet member is likely to increase. In such cases, the degradation in the touch panel""s operability becomes more significant.
In view of the above problems, the present invention aims to provide a glare-resistant touch panel whose appearance and operability are not impaired due to temperature variations, despite its use of a polarizing film in a top sheet member.
The stated object can be fulfilled by a resistive-membrane touch panel including: a first sheet member having a first resistive membrane on one of main surfaces thereof; a second sheet member having flexibility and having a second resistive membrane on one of main surfaces thereof, the second sheet member being opposed to the first sheet member with a spacer interposed in between in such a manner that the first resistive membrane and the second resistive membrane face each other with a uniform spacing therebetween; and a third sheet member being laminated on the outer main surface of the second sheet member, wherein the second sheet member has a polarizing property, and the third sheet member has a lower thermal expansion coefficient than the second sheet member.
With this construction, resistance to glare is ensured by means of the second sheet member that has a polarizing property. Accordingly, when the touch panel is mounted on the front of a display panel such as an LCD panel, the visibility of the content displayed on the display panel is improved. Also, even when the second sheet member is extending to bulge outward as the temperature rises, the third sheet member with a lower thermal expansion coefficient serves to suppress such an outward bulge of the second sheet member, so that the distance between the first and second sheet members will not widen substantially. As a result, the appearance and operability of the touch panel can be maintained regardless of temperature variations.
Here, the second sheet member and the third sheet member may be adhered with a stress relieving adhesive. The adhesive layer formed as such can absorb thermal stress growing between the second and third sheet members at high temperatures. Accordingly, no deviation from alignment will occur between the second and third sheet members, so that the operability of the touch panel can be ensured.
Here, the third sheet member may be coated with an acrylic resin on its surface which is to be adhered to the second sheet member. Accordingly, the adhesion of the third sheet member with the adhesive is strengthened, with it being possible to increase the durability of the touch panel.
Here, the third sheet member may be less hygroscopic than the second sheet member. With this construction, the second sheet member is kept from taking on moisture at high humidities, so that the expansion of the second sheet member due to moisture absorption can be prevented.